This invention relates to processes for alkylating phosphorus-containing compounds. More particularly, this invention relates to processes for alkylating phosphorus-containing compounds possessing at least one phosphorus-hydrogen
alkylatable site to provide an alkylated phosphorus-containing product, e.g., a mono- or dialkylphosphinic acid or metal salt thereof, an alkylarylphosphinic acid or metal salt thereof, an alkyl alkylphosphonic acid or metal salt thereof, an alkyl or aryl dialkylphosphinate ester, an alkyl or aryl alkylarylphosphinate ester, an alkyl or arylalkylphosphorous acid or metal salt thereof or a dialkyl or diaryl alkylphosphonate ester.
In order to prepare dialkylphosphinic acid derivatives, a complex synthesis route has been used which includes the hydrolysis of methyldichlorophosphine to provide methylphosphinic acid and the subsequent esterification of this acid to provide an alkyl methylphosphinic ester. The second phosphorus-hydrogen bond can also be alkylated in the presence of a free-radical initiator.
The use of dialkylphosphinic acid derivatives as flame retardants for polyesters (poly(ethylene terephthalate) and polybutylene terephthalate)) is described in European Patent Publication No. 699,708. These products are synthesized by a complex process using methyldichlorophosphine which is hydrolyzed to methylphosphinic acid; the ester of this acid has been prepared and isolated as intermediate. To prepare dialkylphosphinic acids or derivatives thereof, alkylphosphinic esters can be alkylated by α-olefins at high temperatures under free-radical catalysis conditions. In the case of the reaction of alkylphosphinic acids under the same conditions, only the disproportionation products, namely, alkylphosphines and alkylphosphonic acids, are obtained while under mild conditions no reaction is observed.
U.S. Pat. No. 4,632,741 describes a process for preparing mixtures of salts of alkylphosphinic and dialkylphosphinic acids by reacting an olefin with a salt of hypophosphorous acid in the presence of a photoinitiator using UV light. U.S. Pat. No. 4,590,014 describes a similar process in which the olefin is reacted with the alkali metal salt of hypophosphorous acid in the presence of a free-radical initiator. However, the predominant product monoalkylphosphinic acid.
U.S. Pat. No. 6,300,516 describes a process for preparing dialkylphosphinic acids and/or alkali metal salts thereof by reacting olefins, particularly ethylene, with alkylphosphinic acid or hypophosphorous acid in the presence of azo free-radical initiator. This process is performed in a pressurized reactor with complicated feeding systems which require special safety features. Only high boiling olefins can be reacted with hypophosphorous acid at atmospheric pressure.
The reaction of hypophosphorous acid with isobutene was reported in the literature (S. Deprele and J-L. Montchamp, J. Org. Chem. 2001, 66, 6745). The reaction was run in the presence of a stoichiometric amount of triethyl borane as initiator. The yield of monosubstituted isobutylphosphinic acid was only 38%.